EVIDENCE OF RAPID SEDIMENTATION FROM HIGH-ENERGY FLOWS AND HYPERCONCENTRATED FLOWS IN GLACIFLUVIAL DEPOSITS: SUBAQUEOUS FAN EXAMPLES

High-energy flow and hyperconcentrated flow deposits are becoming more widely recognized in the sedimentary record. In part this stems from an ever-increasing database of field and laboratory studies. Six lithofacies consisting of two gravel facies and four sand facies are described. These facies are grouped into four facies associations. Three of the associations consist of several facies that occur systemically adjacent to one another along a streamwise transect: i) massive gravel to cross-stratified gravelly sand, ii) tabular diffuse-graded sand and low-angle cross-stratified or planar-stratified sand; iii) low-angle cross-stratified sand incised by steep-walled scours and filled with diffusely graded sand. A fourth association that is dominated by climbing ripple-scale cross-lamination is probably more areally widespread and more commonly recognized, yet remains poorly understood.

Strata are interpreted to be high-energy, hyperconcentrated flood-flow deposits emplaced under a regime of rapid flow expansion and loss of transport capacity in a rapidly expanding plane-wall jet with hydraulic jump. Deposition occurred in three distinct streamwise zones: zone of flow establishment, transition zone and zone of established flow. Massive gravels with unconsolidated sand intraclasts and open-work gravel / gravel-sand couplets were deposited in the zone of flow establishment by hyperconcentrated and supercritical flows, respectively. Immediately downflow low-angle cross-stratified sand incised by steep-wall scours infilled by diffuse-graded sand define the transition zone. This zone marks the area of deepest scour erosion related to turbulence generated in migrating hydraulic jumps, and accordingly the distal limit of deposits emplaced under upper-flow-regime conditions. Less well constrained is the relationship between climbing ripple-scale cross-lamination and a possible upflow hydraulic jump.

Data presented suggests that sediment facies attributable to high-energy flows and hyperconcentrated flows are common in esker-subaqueous fans and moraines. Improved understanding of sediment facies will permit continued refinement of landform interpretations and consequently deglacial processes, particularly regarding meltwater events.